going on 15, just tiptoeing from the land of toys to boys, is a tall order made taller still when you're a primordial dwarf? What does that mean to be a primordial dwarf? What is that? It's this thing where it makes you short. She's Normal height, I'm not Normal height. Reporter: Right. Well, what's, what else does it mean? Are you going to stay that tall forever? Yes. Reporter: So this right here will be your height. About. Uh-huh. Pretty much. Reporter: Interesting. At four feet tall and 50 pounds, sienna Bernal -- or sinny, as her family calls her -- is one of only a few hundred people in the world with a genetic condition known as primordial dwarfism, where a person's body, from head to those toes, is in most cases proportional. Just extremely small. The only thing smaller -- are the odds. She would be born with a twin sister, Sierra. How many, how many little people are there? Where? One of the twins is Normal and one is a primordial dwarf? Well, there's none that are twins. Reporter: You're the only ones? Uh-huh. Reporter: At birth, sinny weighed just over a pound and was nearly six weeks behind her sister Sierra in development. Her lungs were not fully developed and she couldn't eat food yet, because everything wasn't developed and her skin was also really, really thin, so, we couldn't touch her. Reporter: The twin's mom, Chrissy. In the early going, what did they say about her chances? Well, you know, the doctor said 10% chance of living. And if she did live, they thought she'd be a vegetable. This is Elvis. Say hi, Elvis! Hi! I like to sing. Reporter: Well, let's just say the good doctors underestimated sinny's spirit. And I like to dance. Reporter: Not that she doesn't have health problems. The bones in her ears, for instance, are so small she needs hearing AIDS. But from the very beginning -- you were a fighter even when you were a tiny, tiny baby. Uh-huh. Yeah, we were born six weeks early. Reporter: Six weeks early. No wonder I'm so short. Reporter: Through the years, the twins were growing up but sinny just wasn't growing. And Sierra was literally becoming the big sister. A role she clearly still loves. Okay. Slow your roll, man. Reporter: When we first met the begin twins, they were 11. Three years later -- Sierra is now nearly double her identical twin's size. When she was younger, actually, they didn't know she was going stop growing, because they didn't really know she was a dwarf until she was older. Reporter: Remarkably, the true cause of sinny's size remained undiagnosed for years. Chrissy and the twin's adoptive father, joey, say doctors didn't even suggest primordial dwarfism until sinny was 6. Proportionally, she was growing. Right, she was growing appropriately. Reporter: Arms, legs -- Just, you know, honey, I shrunk the kids. Reporter: You guys have an amazingly honest sort of approach to things. Well -- It's the only way to be. Life is the way it is. You either choose to deal with it or -- Reporter: Well, apparently both sinny and Sierra have picked up on that. You could fit in my locker, couldn't you? Yeah. Reporter: Because they speak very candid about everything. Oh, she'll walk in the house, "Dwarf in the house!" There's no other way to treat it. If this is the roughest here at home, she'll be able to handle the world. Reporter: And handle it she has. The last time we saw them -- I'm going to throw out a word now and I'd just like to have you react to it. Boys? Reporter: Sinny seemed to be holding her own. That's you. Once up had four crushes at a time and I didn't even have one. Reporter: Well, we found things really haven't changed. At least in the boyfriend department. She's had one before I do. I don't care though. I had three. Reporter: But sinny is the first to tell you, the transition from dwarf tween to dwarf teen hasn't been without growing pains. Like, man, it sucks. You know, the world's not built for me. Reporter: Sinny has had to watch her sister start driving lessons -- from the back seat -- still buckled into a booster chair. And though they once shared the dream of a singing career -- it is Sierra who is now on stage, writing and performing her own music. Sienna started acting out really bad. I was like, why are you so angry all the time? And she said, because Sierra is living my dream. What we did, after that particular day, was really try and figure out things that she could do. Reporter: Like joining a special needs cheering squad. Down and jump. Down and jump. Reporter: And finding new hobbies, like cooking. Still -- sinny's hearing and vision have worsened since ore first interview. And she's developed scoliosis and battles some learning disabilities. I'm in a lot of pain. But I'm worth it. Reporter: And though she's defied everybody's expectations, I know my kids won't like them, though. Reporter: Sinny's long-term stealth an unknown. How do you prepare your heart for something like that? I mean, how do you do that? Just try not to focus on that and move forward. Reporter: A lot of people are going watch this on television. What would you want them to know about you guys? That when you see someone little, you should just probably help them if they need help. Make them feel comfortable about themselves. Make them feel happy. Little people rule! Clap, up, down and jump. You have to find what gift you've been given, focus on that, instead of focusing on what you don't have. The sky is always blue in her world, and she will make sure that your sky is always

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{"id":22421644,"title":"Life as a Teenage Primordial Dwarf With a Twin","duration":"3:00","description":"Act 3: Sinny and Sierra Bernal are the only known identical twins with a primordial dwarf.","url":"/2020/video/life-teenage-primordial-dwarf-twin-22421644","section":"2020","mediaType":"Default"}